This invention relates to apparatus for forming an end finish on a hollow article having a plastically deformable tubular end portion terminating in an open end of the article. The invention also relates to a membrane for use in such apparatus.
Ordinarily in the production of plastic containers of the type which includes bottles and jars, and particularly those intended to be provided with a closure, a so-called end finish or neck finish is formed on the outer surface of the container adjacent to its open end. The configuration of each neck finish is dictated by such considerations as the type and form of the closure selected for the container, sealing requirements, desired ease of handling, compatibility with filling equipment, and the like. In many cases the neck finish must be formed with comparatively great precision in order to meet the criteria imposed by such considerations.
In one system of blow-molding plastic containers, a tubular preform or parison of thermoplastic material is brought to a temperature at which it is plastically deformable and placed in a mold cavity having the configuration of the finished container. The wall of the parison is then subjected to a fluid pressure differential whereby the parison is expanded to conform to the cavity surface. The parison may be one that has been injection-molded with one end closed, or it may be an extruded tube cut to length, one end of which is closed mechanically before the blow-molding operation is performed. In the case of injection-molded parisons, the neck finish can be formed during the injection-molding process, but this adds substantially to the cost of the injection-molding equipment. Further, some materials are subject to significant shrinkage is reheated prior to the blow-molding operation. It is impracticable to form the neck finish on an extruded parison during extrusion. In view of these considerations, the neck finish is frequently formed in the well-known manner illustrated in FIGS. 1 and 2 of the accompanying drawing, usually just prior to expansion of the parison.
More particularly an end portion of a plastically deformable parison 10 is received within a neck mold 12 which comprises mold members 14, 16 and 18. Mold members 14 and 16 are movable to and from the closed position of the mold shown in FIGS. 1 and 2 in which they cooperate to define a neck mold surface 20. Adjacent to the mold surface is a choke ring 22 which partially penetrates the wall 24 of the parison when mold members 14 and 16 are moved to their closed position, as shown. The choke ring must be so configured that it firmly constrains the parison against axial movement in the upward direction as viewed in the drawing. Mold member 18 is provided with an annular shoulder or ledge 26 for engagement with the rim 28 which surrounds the open end of parison 10. Concentric with ledge 26 and surrounded thereby is an aperture 30 extending through mold member 18. Aligned with aperture 30 and adapted to be received therethrough is a swage 36, usually formed of metal and having a cylindrical body 38 and a frustoconical cam end 40. The maximum transverse dimension of the swage is somewhat larger than that of the inner surface 32 of parison 10 but smaller than that of its outer surface 34.
In forming an end finish on parison 10, which end finish will become the neck finish of the container to be blow-molded, swage 36 is moved upwardly from the position shown in FIG. 1 to that shown in FIG. 2. During such movement, the swage cam end 40 acts to force the material of parison wall 24 laterally outwardly to conform outer surface 34 to neck mold surface 20.
The conventional swaging procedure illustrated in FIGS. 1 and 2 is quite satisfactory in many applications. However, when fine precision and rigidly consistent results are called for, it is inadequate, or in some cases wholly unacceptable, for several reasons.
Firstly, when cam end 40 has passed beyond the end portion of the parison; that is, when swage 36 has arrived at the position shown in FIG. 2, and as the end portion begins to cool in the neck mold, its wall may tend to shrink somewhat. If sufficient shrinkage occurs, the compressive forces exerted on the wall by the swage and neck mold surface 20 will be reduced to zero, and the outer surface of the end portion may therefore tend to draw away from neck mold surface 20. While this tendency can be anticipated in designing the neck mold surface, consistent results and precise conformation to neck design are extremely difficult if not impossible to achieve.
Secondly, parison rim 28 and adjacent portions of inner surface 32 are necessarily deformed as the result of frictional forces generated at the interface of the surface of the swage and the inner parison surface. Referring to FIG. 3, these forces are shown as an axial component F.sub.A of the force F applied to the parison wall by cam end 40 of the swage, vector F.sub.R representing the radial or lateral component acting to conform the parison wall to the neck mold surface. While the axial component F.sub.A is present to some degree in all instances, it increases in magnitude as the temperature of the swage rises during continuous use. It also increases as parison materials of higher viscosity are substituted, and it becomes excessive in the case of certain thermoplastics, notably polyethylene terephthalate and the high acrylonitriles (which include acrylonitrile as a major constituent and lesser amounts of styrene and sometimes butadiene). In any event, some of the material of parison wall 24 is drawn with the swage in the axial direction to enlarge or flare the open end of the parison, as shown at 42 in FIG. 2, thereby deforming rim 28 and diminishing its area. As a result, the rim may not cooperate adequately with the container closure, especially if a good seal therebetween is required.
Various expedients intended to alleviate this problem have been attempted heretofore but each has its own deficiencies. Thus, if a lubricant such as a silicon preparation is applied to the surface of the swage, the cost of the lubricant and its applicator must be considered. Further, it has been found that it is difficult to apply a surface lubricant uniformly, and even if only a small area is missed, material at the inner surface of the parison is nevertheless drawn along with the swage. If the container is to be filled with a food, a beverage, a medicinal or therapeutic preparation, or any other substance requiring a sterile or sanitary package, the lubricant becomes a contaminant which must be removed completely before the container is filled, thereby adding to the filler's costs and creating a potential for liability. Similar considerations apply in the case of other means of lubrication, as by impregnating a porous swage with graphite.
If the swage is coated with a material having a low coefficient of friction, graphite or Teflon for example, the coating wears rapidly. Attempts to form the swage entirely of a low-friction material such as Teflon or other synthetic resin have resulted in early plastic deformation of the swage.
Finally, the conventional swaging operation may not be satisfactory in the case of a neck finish which calls for relatively large lateral displacement of the parison material; that is, one which requires a thread, a bead, a flange or the like to be executed in high relief. In such a case, necessary limitations on the dimensions of the swage, parison, and neck mold surface render it difficult or impossible to displace the material of the parison wall sufficiently to conform its outer surface to neck mold surface 20.